Purpose: Sprint interval training (SIT) provides a potent stimulus for improving maximal aerobic capacity (V̇O2max), which is among the strongest markers for future cardiovascular health and premature mortality. Cycling-based SIT protocols involving six or more ‘all-out’ 30-s Wingate sprints per training session improve V̇O2max, but we have recently demonstrated that similar improvements in V̇O2max can be achieved with as few as two 20-s sprints. This suggests that the volume of sprint-exercise has limited influence on subsequent training adaptations. Therefore, the aim of the present study was to examine whether a single 20-s cycle-sprint per training session can provide a sufficient stimulus for improving V̇O2max.

Methods: Thirty sedentary or recreationally active participants (10 men / 20 women; mean±SD age: 24±6 y, BMI: 22.6±4.0 kg·m-2, V̇O2max: 33±7 mL·kg-1·min-1) were randomised to a training group or a no-intervention control group. Training involved three exercise sessions per week for four weeks, consisting of a single 20-s Wingate sprint (no warm-up or cool-down). V̇O2max was determined prior to training and three days following the final training session.

Results: Mean V̇O2max did not significantly change in the training group (2.15±0.62 vs. 2.22±0.64 L·min-1) or the control group (2.07±0.69 vs. 2.08±0.68 L·min-1; effect of time: P=0.17; group ´ time interaction effect: P=0.26).

Conclusion: Although we have previously demonstrated that regularly performing two repeated 20-s ‘all-out’ cycle-sprints provides a sufficient training stimulus for a robust increase in V̇O2max, our present study suggests that this is not the case when training sessions are limited to a single sprint.

In cross-sectional studies, maximal aerobic capacity (V̇O2max) is one of the strongest prognostic markers for future cardiovascular health and premature death (Myers et al. 2002, Keteyian et al. 2008). Moreover, improving V̇O2max is associated with substantial reductions in the risk for mortality during follow-up in longitudinal studies (Blair et al. 1995, Lee et al. 2011). Regular physical activity (PA) is the only feasible means of improving absolute V̇O2max, but the association between PA levels and mortality disappears after adjustment for V̇O2max (Lee et al. 2011), suggesting that a high V̇O2max is more important than high PA levels. Thus, it has been recommended that besides encouraging reductions in sedentary time and increases in overall PA, improving V̇O2max should also be a key public health message (Lee et al. 2011, Bouchard et al. 2015).

PA guidelines based on moderate-intensity aerobic exercise have been consistently promoted for over two decades (Pate et al. 1995, Garber et al. 2011), but the adherence to these recommendations remains poor in the general population (Hallal et al. 2012). In order to address the commonly reported barrier of lack of time (Korkiakangas et al. 2009), submaximal high-intensity interval training (HIIT) and supramaximal sprint interval training (SIT) have been proposed as time-efficient alternative/adjunct exercise strategies (Gillen and Gibala 2014). A common type of SIT protocol consists of 4-10 repeated 30-s ‘all-out’ Wingate sprints, thus resulting in just 2-5 minutes of high-intensity exercise per session (Weston et al. 2014). Such protocols have been shown to provide a robust increase in V̇O2max, superior to that following aerobic exercise training (Burgomaster et al. 2008, Bailey et al. 2009, Macpherson et al. 2011, Sandvei et al. 2012, Nalcakan 2014, Milanovic et al. 2015). However, the low volume of high-intensity exercise does not necessarily result in a time-efficient exercise intervention per se, as the need for recovery periods in between sprints generally results in a total training time commitment in excess of 30 min per session (Gillen and Gibala 2014).

Because the mechanisms by which SIT improves V̇O2max are poorly understood, it also remains unknown how the training stimulus can be optimised in order to achieve either the largest increases in V̇O2max, or a set increase using the smallest amount of time and effort. However, recent evidence suggests that Wingate-based SIT protocols can be made shorter and less strenuous while retaining the positive effect on V̇O2max. Two studies have directly compared the effects of reducing sprint duration from 30 s to either 10 s (Hazell et al. 2010) or 15 s (Zelt et al. 2014), and neither study observed a lower increase in V̇O2max with the shorter sprint duration. Furthermore, four training studies have examined the effect on V̇O2max of SIT protocols incorporating fewer than 4 supramaximal sprints per session. SIT protocols consisting of three 20-s sprints (Gillen et al. 2014) or three 30-s sprints (Allemeier et al. 1994, Ijichi et al. 2015) were reported to increase V̇O2max by 12%-14%. Moreover, in our lab we observed a mean increase of 14% (Metcalfe et al. 2012) following a SIT protocol with just two 20-s all-out sprints. Changes of such magnitude favourably compare with more strenuous SIT protocols: recent meta-analyses have reported a range of improvements in V̇O2max of 3-14% for SIT studies involving 4-10 repeated Wingate sprints per session (Sloth et al. 2013, Gist et al. 2014, Weston et al. 2014).

The fact that performing fewer and/or shorter supramaximal sprints is sufficient for improving V̇O2max suggests that the total volume of high-intensity exercise is not a key determinant of the training stimulus. Conversely, it is plausible to hypothesise that the training stimulus resides predominantly within the first of repeated sprints. If this is indeed the case, then an exercise training protocol consisting of a single supramaximal sprint per session should be sufficient to increase V̇O2max. There is some mechanistic support for this hypothesis: the signalling molecule AMPK, which is deemed important for aerobic adaptations (Gibala et al. 2012), is regulated by glycogen availability (McBride et al. 2009). It has been shown that glycogen depletion during repeated supramaximal sprints is limited to the first sprint (Parolin et al. 1999), and AMPK activation has been observed in response to a single Wingate sprint (Guerra et al. 2010, Fuentes et al. 2012).

Considering the strong association between V̇O2max and health, and the fact that lack of time is consistently reported as an important barrier to performing sufficient exercise, there is a need to identify the lowest volume of exercise effective at modifying V̇O2max. Thus, the aim of the present randomised controlled trial was to determine whether regularly performing a single 20-s ‘all-out’ cycle sprint provides a sufficient training stimulus for improving V̇O2max in sedentary or recreationally active individuals.

METHODS

Compliance with Ethical Standards

The study was approved by the local University Ethics committees (reference: EP 14/15 87 / FC272014-2015), and conformed to the standards set forth in the latest revision of the Declaration of Helsinki. The study protocol was fully explained to all participants in written and verbal form before they were asked to provide written consent.

Participants

Thirty apparently healthy, sedentary or recreationally active participants (10 men / 20 women; mean±SD age 24±6 y, BMI 22.6±4.0 kg·m-2, V̇O2max 33±7 mL·kg-1·min-1) were recruited at three sites in the UK (Bath, Worcester, Derry/Londonderry) and randomised into a training group (n=15; 5 men) and a control group (n=15; 5 men). Exclusion criteria were classification as highly physically active according to the International Physical Activity Questionnaire (IPAQ (Craig et al. 2003)), contraindications to exercise as determined using a standard physical activity readiness questionnaire (PAR-Q (Thomas et al. 1992)), clinically significant hypertension (>140/90 mm Hg), or resting heart rate ≥100 bpm. Using the IPAQ, the activity level of 16 participants was categorised as ‘low’, and the activity level of the remaining 14 participants as ‘moderate’.

Experimental Procedures

An incremental cycling test to exhaustion was performed on an electronically-braked ergometer in order to determine V̇O2max (Excalibur Sport / Corival, Lode, Groningen, the Netherlands). Participants were asked not to perform strenuous exercise or consume caffeine or alcohol the day before and prior to the test, and to drink half a litre of water the morning of the testing day. Participants completed a 2-min warm-up at 50 W after which the intensity was increased by 1 W every 3 s until volitional exhaustion despite verbal encouragement. Oxygen uptake (V̇O2) was determined throughout the test using an online gas analyser (TrueOne 2400, Parvo Medics, Sandy, UT, US; COSMED Quark CPET, Rome, Italy; Oxycon Pro, Jaeger, Wurzburg, Germany) to determine V̇O2max as the highest value for a 15-breath rolling average. Values for V̇O2max were accepted if two or more of the following criteria were met: 1) volitional exhaustion, 2) RER>1.15, and 3) maximal heart rate within 10 beats of the age-predicted maximum (i.e. 220-age). This was the case for all participants, except for one control participant who was excluded from the data analysis (inclusion of this participant did not alter the statistical results).

Following the V̇O2max test, participants in the training group started a 4-week training programme consisting of 3 training sessions per week. Training sessions involved a single 20-s ‘all-out’ adapted Wingate sprint against a resistance equivalent to 7.5% of the participant’s pre-training body mass. Participants were asked to achieve the highest pedal frequency possible during a ~1-2 s unloaded lead-in prior to applying the full resistance. Participants then received strong verbal encouragement to maintain the highest pedal frequency throughout the remaining 18-19 s. In order to allow us to specifically investigate the effects of the 20-s sprints, no warm-up or cool-down were performed. In order to be included in the final data analyses, participants could not miss more than 2 training sessions, 2 training sessions within the final week, or the final training session. No participants failed to meet these criteria. Participants allocated to the control group were asked to maintain their usual physical activity patterns for the duration of the study. All participants were asked not to change their current eating behaviour.

Acute exercise-induced changes in plasma volume were determined during the first and last training sessions. After at least 15 min of seated rest, a finger-prick blood sample was taken and directly analysed for haemoglobin (Hb) levels (HemoCue, Crawley, UK). A sample for determination of haematocrit (Hct) was stored for analysis after all subsequent samples had been collected (Haematospin 1300; Hawksley & Son Ltd, Lancing, UK). A second sample was taken directly after completion of the sprint, with further samples taken at rest in a seated position at 3 min, 10 min and 30 min following the end of the sprint. Plasma volume changes were calculated as described by Dill & Costill (1974). Peak (PPO), end (EPO), and mean power output (MPO), as well as peak heart rate, were recorded during the 3rd and 12th training sessions. Rating of perceived exertion (RPE (Borg 1970)) was determined directly after the 3rd, 6th, 9th and 12th training sessions.

A second V̇O2max test was performed three days after the final training session, at a similar time as the baseline test and following identical procedures. Participants in the control group performed their second V̇O2max test after a similar duration compared to participants in the training group. For the day before testing and on the testing day itself, participants were asked to follow a diet similar to that for the baseline test.

Statistical analysis

Data are presented as mean±SD. Based on a coefficient of variation of the V̇O2max test protocol of 4%, it was calculated that 14 participants were needed in each group in order to be able to detect a difference in the change in V̇O2max of 5% between the training group and the control group, with a power of 90% and α=0.05. Two-way mixed model ANOVAs (group x time) were performed to determine differences in the change in Wmax and V̇O2max from baseline to post-intervention between the training group and the control group. Two-way repeated measures ANOVAs (training session x time) were used to assess the effect of acute exercise on plasma volume change. Differences in peak HR, PPO, MPO and EPO between the 3rd and 12th training sessions were determined using paired-sample t-tests. Alpha was set at 0.05.

RESULTS

There were no significant differences in mean baseline characteristics between participants in the control group and the training group (Control - age: 23±5 y, BMI: 22.4±3.5 kg·m-2, V̇O2max: 32±6 mL·kg·min-1; Training - age: 24±6 y, BMI: 22.9±4.5 kg·m-2, V̇O2max: 34±8 mL·kg·min-1). Body mass did not change from baseline to reassessment in the training group (63.6±15.6 vs. 63.9±14.9 kg) or in the control group (64.4±12.8 vs. 64.4±12.8 kg). Of the fifteen participants in the training group, twelve completed all 12 training sessions, two completed 11 sessions, and one completed 10 sessions, resulting in an overall mean adherence of 98%. Characteristics of the training sessions are provided in Table 1. Peak and mean power output were not significantly different between the third and twelfth training sessions, but end power output was increased by 9% in the 12th compared to the 3rd session (P=0.03). Peak heart rate reached 90±11% and 91±4% of HRmax during the 3rd and 12th training sessions respectively. Plasma volume was significantly reduced throughout the post-exercise period (P=0.02), with no difference between the post-exercise time-points or between sessions 1 and 12 (Table 1).

Maximal power output (Wmax) was increased in the training group (185±50 vs. 195±50 W) compared to the control group (180±48 vs. 174±43 W; group ´ time interaction effect: P=0.001). However, mean V̇O2max did not significantly change from baseline in the training group (2.15±0.62 vs. 2.22±0.64 L·min-1) or the control group (2.07±0.69 vs. 2.08±0.68 L·min-1; effect of time: P=0.17; group ´ time interaction effect: P=0.26). There were no significant correlations between the change in V̇O2max and either physical activity levels as measured using the International Physical Activity Questionnaire (R2=0.06), or baseline V̇O2max (R2=0.00). Interindividual variability in the change in V̇O2max was larger in the training group (range: -10% to +21%) compared to the control group (-9% to +7%; Figure 1).